This applications relates to wired and wireless communications including communications based on, among others, OFDM (Orthogonal Frequency Division Multiplexing), OFDMA (Orthogonal Frequency Division Multiple Access), and SC-FDMA (Single Carrier Frequency-Division Multiple Access) systems.
In various wireless cellular networks, communication capacity and data throughput may be degraded due to unavailable or congested network spectrum. Allocated spectral bands are becoming increasingly congested with desired and undesired signals due to the proliferation of both intentional and unintentional electromagnetic emissions. Such a congested spectrum can lead to signal degradation and interferences. For example, both low and high power signals may be simultaneously observed by a receiver's antenna or antenna array. Under such conditions, desired signals may be obscured and undetectable since they can be buried beneath much stronger clusters of interfering signals.
Among the different technologies that can make use of the spectrum, Orthogonal Frequency Division Multiplexing (OFDM) is a technique for multicarrier data transmission that has been standardized for several wireless network physical layers. In OFDM, an allocated channel is divided into a number of orthogonal subchannels. Each subchannel has an equal bandwidth and is made of a unique group of subcarrier signals. The subcarrier signals are orthogonal in that the inner product of any two of the subcarriers equals zero. The frequencies of the orthogonal subcarrier signals are equally and minimally spaced so data modulation of the subcarrier signals facilitates optimal bandwidth efficiency. In comparison, frequency division multiplexing for multicarrier data transmission utilizes non-orthogonal subcarrier signals and uses segments of allocated channel bandwidth to isolate subcarrier signal frequency spectra.
Orthogonal Frequency Division Multiple Access (OFDMA) is a multi-user version of OFDM technology. The multiple access is achieved in OFDMA by assigning subsets of orthogonal subcarriers to individual subscriber stations. OFDMA may be viewed as a combination of frequency domain and time domain multiple access where radio resources are partitioned in a time-frequency space, and network user data bursts are assigned along the OFDM symbol index as well as OFDM sub-carrier index. OFDMA has been widely adopted by various standard bodies.
The Single Carrier Frequency Division Multiple Access (SC-FDMA) can be viewed as either a linearly precoded OFDMA scheme, or a single carrier multiple access scheme. One advantage of SC-FDMA over a conventional OFDMA is that the SC-FDMA signal has lower peak-to-average power ratio (PAPR) because of its inherent single carrier modulation method. The SC-FDMA can also be considered as an alternative to OFDMA, especially for the uplink communications where lower PAPR benefits the mobile terminal power efficiency. SC-FDMA has been adopted for uplink multiple access scheme in 3GPP Long Term Evolution (LTE), or Evolved UTRA.
In the frequency domain, an OFDM or OFDMA signal is made up of orthogonal subcarriers, the number of which determines the size of the Fast Fourier Transform (FFT), NFFT. FIG. 1A illustrates the OFDMA bandwidth definition. Assuming Δf is the subcarrier spacing, the sampling frequency fS can be calculated with the formula:fS=Δf×NFFT For a given nominal channel bandwidth BW, only a subset of subcarriers NSIG out of NFFT is occupied for signals, referred as signal bandwidth BWSIG. NSIG may include DC sub-carrier which often contains no data. The rest of the subcarriers which are not used for transmission of data and information serve as guard subcarriers. The guard subcarriers are used to enable the signal to naturally decay and create the FFT “brick wall” shaping. The rule of thumb to select the FFT size is to choose the smallest power of two that is greater than NSIG. As illustrated, the normal channel bandwidth BW is greater than the signal bandwidth due to the presence of the guard subcarriers on both sides of the signal-carrying subcarriers. The sampling frequency fs is selected to be greater than the normal channel bandwidth.
In the OFDMA physical layer, the resource grid and basic resource block are defined. Based on the defined resource grid, one or multiple basic blocks in a group in the frequency domain are defined as a subchannel in some standards. NSIG may contain multiple subchannels or basic resource blocks, each consists of NSC subcarriers. The subchannel is used as the minimum channel bandwidth division unit in this document and each subchannel has NSC subcarriers.
The Inverse Fast Fourier Transform (IFFT) creates an OFDM or OFDMA waveform and the associated time duration is referred to as the useful symbol time TIFFT. FIG. 1B illustrates the time domain symbol structure of an OFDM or OFDMA signal. A copy of the last of the useful symbol period is known as the Cyclic Prefix (CP) TG and is used to collect multipath, while maintaining the orthogonality of the tones. In addition, a small windowing period can be optionally added to a time slot before the CP and a time slot at the end of a symbol time. Adding windowing periods can reduce the signal in-band emission and the signal out-of-band emission. The total symbol time TSYM includes the additional CP time TG, and optional windowing time TWIN, TSYM=TG+TIFFT+TWIN. Using a cyclic extension, the samples required for performing the FFT at the receiver can be taken anywhere over the length of the extended symbol. This provides the multipath immunity as well as a tolerance for symbol time synchronization errors.